There is a well-recognized need to produce duplicate copies of magnetically recorded signals, such as with video cassettes of motion pictures and educational materials. Production of small numbers of duplicate copies of magnetically recorded tapes is often performed by simply playing the original tape into a second tape recorder. If larger numbers of copies are needed, however, a process called contact duplication can be faster and more efficient.
Contact duplication involves creating an original recording, called a master, which is then placed in direct contact with a recording medium, such as a blank recording tape. Once the two media are in contact, a transfer means, such as heat or a magnetic field, is applied to the combined media, which causes the magnetically recorded signals on the master to produce a magnetic impression of one of the original signals on the recording medium.
If the transfer means is heat, the process is commonly called "thermomagnetic duplication" or "thermoremanent duplication." In thermomagnetic duplication, a recording medium such as chromium dioxide, chosen because of its low Curie temperature, is used as the recording medium. By quickly heating the recording medium above its Curie temperature, and then cooling it below its Curie temperature while maintaining it in intimate contact with the master, the magnetic fields due to the signals recorded on the master tape induce similar fields in the recording tape. A process and apparatus for carrying out duplication of magnetic tapes by this method, using a laser beam as the heat source, is disclosed in U.S. Pat. No. 4,631,602.
Signals also can be transferred from the master to the recording media by applying a strong magnetic field to the combination of the two contacting media. A tape duplicating system which uses this signal transfer method is disclosed in U.S. Pat. No. 5,148,403.
In a typical manufacturing process for duplicating programs for videocassettes, several copies of a program are contact duplicated onto a length of tape, which is then rolled into a large roll, called a pancake, for loading into cassettes. Referring to FIG. 1, tape 10 typically contains a leading length 12, followed by a segment 14 which contains a recorded program, followed by segment 16, which contains recorded signal, called a cue tone 18, followed by second recorded program 14. Additional segments and cue tones may follow.
To make a cassette, the pancake is placed in a cassette loading apparatus, and a cassette, having a leader attached to a supply reel is also placed in the loading apparatus. A program segment 14 is attached to the leader and is wound onto the cassette reel. When the loading apparatus senses the signal from the cue tone 18 following this program segment, it stops the winding, cuts the tape 10, and splices the trailing end of the tape (at the end of the segment 14) onto a length of transparent film attached to the takeup reel in the cassette being loaded.
A troublesome problem in some cassette loading processes is cue tone error caused by remanent magnetization in the recording tape. Such remanent magnetization is typically the result of magnetic orientation fields applied to the magnetic recording medium during manufacture as disclosed, for example, in U.S. Pat. No. 2,796,359. Cue tone error can lead to a condition where the tape cutting operation loses synchronization with the programs recorded on the tape, resulting in a cassette containing the end of one program followed by the beginning of another.
Demagnetization, or erasing, of recording tape is well known, since it is commonly done in magnetic tape recording apparatus, where magnetic tape is frequently recorded over, and it is desirable to erase the old signals prior to recording the new.
A simple apparatus for erasing magnetic recordings in a tape recorder is disclosed in U.S. Pat. No. 2,535,498. Two permanent bar magnets are used to magnetize the recording medium to alternating polarities, with fields of progressively diminishing strength.
A direct-current, electromagnetic erase head, which first subjects the medium to a saturating magnetic field of one polarity and then applies a field of reverse polarity of such magnitude to just bring the medium to zero magnetization, is disclosed in published British Patent Application No. GB 2 195 198.
An apparatus for erasing an entire roll of tape contained in a cassette, without unrolling it, is disclosed in U.S. Pat. No. 5,204,801. The cassette is conveyed through a strong rotating magnetic field which subjects the tape to varying magnetic fields of varying directions. This method of tape demagnetization is sometimes called "bulk degaussing" or "bulk erasure."
A process for removal of remanent magnetization in the sound recording layer to reduce noise in a motion picture film strip is disclosed in U.S. Pat. No. 3,627,580. A series of permanent magnets of alternating polarity, spaced at increasing distance from the medium, is used to demagnetize the magnetic recording layer by subjecting it to magnetic fields of alternating polarity and progressively diminishing strength.
While these devices can demagnetize or erase magnetic tape, they present certain difficulties when it is attempted to apply them, in a cost-effective way, to the demagnetization of recording of media for contact duplication.
In particular, since magnetic recording tape is typically produced as a wide web of magnetic medium which is then slit into tapes, the simplest and most cost-effective place to perform the demagnetization is before slitting. This necessitates a demagnetization apparatus capable of applying a demagnetizing field across a web of material many times the width of a single tape. Strong permanent magnets of large size are expensive, and the resulting apparatus is likely to be extremely heavy and difficult to install in a manufacturing apparatus.
In addition, it is strongly desired that demagnetizing devices used as part of a magnetic media manufacturing operation be of the permanent magnet type, to avoid the expense and hazard of electrical devices. There is a need for an alternative method to demagnetize tape which will be effective against the type of cue tone error described above.